The overall aim of the proposal is to develop an effective live vaccine to prevent rabbit pasteurellosis. The major serotypes of rabbit P. multocida isolates, collected from several parts of the country, will be identified. P. multocida isolates of the most predominant serotypes will be selected to generate mutants, and these mutants will be used to produce live vaccines. Four types of live vaccines will be evaluated, i.e., avirulent mutant vaccines, streptomycin-dependent mutant vaccines, temperature-sensitive mutant vaccines, and streptomycin-dependent - temperature-sensitive double mutant vaccines. Immune mechanism(s) associated with vaccine protection will be studied. Humoral antibody responses will be measureed by enzyme-linked immunosorbent assay (ELISA), neutrophil phagocytosis and killing assay, and alveolar macrophage phagocytosis and killing assay. Presence or absence, titer, and class of ELISA antibodies in sera and bronchial secretions will be determined and correlated with the results of passive transfer experiments with sera or bronchial secretions. Opsonic antibody activity in sera will be assayed with a neutrophil phagocytosis and killing assay, and the results correlated with passive transfer experiments of sera. Similarly, opsonic antibody activity in bronchial secretions will be assayed with an alveolar macrophage phagocytosis and killing assay, and the results correlated with passive transfer experiments of bronchial secretions.